1. Field of the Invention
The present invention relates to methods for determining a coordinate system for a device under measurement and to coordinate measuring apparatuses. More particularly, the present invention relates to a method for setting a coordinate system for a device under measurement both easily and accurately.
2. Prior Art
Conventionally, coordinate measuring apparatuses, such as surface-characteristic measuring apparatuses have been used for measuring surface characteristics of a device under measurement, such as surface roughness and surface shapes.
Coordinate measuring apparatuses can automatically move along each of their axes and perform measurement under computer numerical control with the use of part programs that specify measurement procedures, including measurement areas and movement instructions for the apparatuses.
Instructions in the part programs are described either in a machine coordinate system determined by a unique origin of the coordinate measuring apparatus or in a coordinate system for the device under measurement, determined by the origin of the device under measurement.
An instruction for setting a coordinate system for a device under measurement is frequently used in a part program because it allows a coordinate measuring apparatus that uses the part program to perform automatic measurement with the coordinate system for the device under measurement being changed.
Conventionally, to set a coordinate system for a device under measurement, a convenient measurement unit is frequently used instead of a detector of the coordinate measuring apparatus. In that case, the shape of almost the entire device under measurement, including the external shape and the internal shape of the device under measurement, is measured first, and then, a coordinate system for the device under measurement is set.
In a conventional case, for example, the external shape of almost the entire device under measurement is measured, and then the origin of a coordinate system for the device under measurement is specified (this case is disclosed, for example, in Japanese Patent Applications No. 2002-270307 and No. 2002-340930).
In another conventional case, the shapes of almost the whole of the internal parts of a plurality of holes provided for a device under measurement are measured, and then a coordinate system for the device under measurement is specified (this case is disclosed, for example, in Japanese Unexamined Patent Application Publications No. Hei-7-270152 and No. Hei-7-128044).
The shape information of a device under measurement can be obtained by applying image processing to the result of shape measurement (this case is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-339478).
Since a coordinate system is individually set for each device under measurement in a part program, the setting operation must be performed many times. Therefore, there are demands for an apparatus that can easily set the coordinate system for a device under measurement, and it is also preferred that the setting be performed as accurately as possible.
However, there is conventionally no suitable technology which allows a coordinate system for a device under measurement to be set both easily and accurately.
Conventionally, if the number of measurement points used for obtaining the shape information of a device under measurement is reduced in order to more easily set a coordinate system for the device under measurement, the coordinate system for the device under measurement cannot be accurately set in some cases. Therefore, the shape of almost the entire device under measurement is usually measured.
Conversely, if the number of measurement points is increased in order to more accurately set a coordinate system for a device under measurement, the measurement requires more time, which is troublesome. When the setting is performed many times, this issue becomes more problematic.